1. Field of the Invention
This invention relates to an object information acquisition apparatus such as an image acquisition apparatus and also to an object information acquisition method for acquiring information on an object by means of an electromagnetic wave. More particularly, the present invention relates to an image acquiring apparatus and an image acquisition method for observing the properties and the profile of an object by means of a high frequency electromagnetic wave in the domain of millimeter waves and terahertz waves (30 GHz to 30 THz).
2. Description of the Related Art
Non-destructive sensing technologies using electromagnetic waves within a range from millimeter waves to terahertz (THz)-waves (in a frequency range between 30 GHz to 30 THz, to be also referred to simply as “terahertz (THz) waves” hereafter) have been developed in recent years. Applicable fields of electromagnetic waves of the above-cited frequency band include imaging techniques for seeing-through inspections without using X-rays. Spectrometric techniques for looking into physical properties of a substance by determining the absorption spectrum and the complex permittivity of the inside of the substance, techniques for analyzing biomolecules and those for evaluating the carrier density and the mobility of a substance have also been developed.
Additionally, studies are under way to install seeing-through inspection apparatus that employs THz waves at customs inspection gates of airports in order to detect prohibited drugs and dangerous articles hidden in bags, clothes that passengers are wearing and human bodies. Particularly, techniques employing THz waves are believed to be effective for human bodies because inspections involving irradiation of X-rays entail the problem of exposure to harmful radiation. As a proposal for such a technique, a detector for determining if any prohibited drug is hidden in a human body or not by irradiating the human body with a THz wave and monitoring reflected rays has been disclosed.
As disclosed in Japanese Patent Application Laid-Open No. 2005-265793, a photoconductive switching device that is equipped with an antenna arranged on a photoconductive film formed on a substrate so as also to operate as an electrode can suitably be used as a THz wave generation unit. LT-GaAs made to grow on a GaAs substrate at low temperature can be used as a photoconductive film. CO2 lasers and QC (quantum cascade) lasers can also be utilized as THz wave generations sources. However, the latter lasers cannot acquire an absorption spectrum that is specific to the object of inspection and extends over a wide frequency range, and hence the use of a plurality of THz wave generation sources of different wavelengths is necessary.